The present invention relates to potentiometers of the type used in fuel gauges for measuring the amount of fuel in a fuel tank of a vehicle and in particular to improved potentiometer.
A variable resistor card having a card terminal, a plurality of contacts with resistors between the contacts, and a wiper track is part of a fuel sensor for a motor vehicle. Associated with the card is a wiper which is movable along the contacts of the wiper track and is connected to a float which moves horizontally in response to changes in the level of the fuel within the tank. The resistor is configured such that a change in the volume of fuel within the tank is proportional to the change in current passing through the resistor. The resistor is connected by a battery to an ammeter on the dashboard which serves as a fuel gauge.
Existing variable resistor cards employed in fuel gauges of vehicles have a non-conductive substrate upon which are deposited a plurality of elongate conductive lands each of which has a contact end and a resistor end. The lands are oriented to be generally parallel to each other with resistors positioned between the resistor ends of the lands and the contact ends are configured to form the wiper track across which the wiper is movable. The lands are formed by the deposition of conductive material upon the non-conductive substrate of the resistor card, and the lands have a thickness such that when seen in cross-section the lands stand as humps above the surface of the non-conductive substrate of the resistor card. As the wiper moves along the wiper track at the contact ends, the wiper moves across the humps of the lands and across the valleys between the lands. Over a long period of time, typically several years, movement of the wiper across the wiper track will cause the conductive material of the lands to be worn away. The wearing of the material of the lands will eventually cause the failure of the potentiometer and require replacement of the resistor card. It would be helpful to provide a resistor card which would have an extended life, that is, a resistor card having conductive lands which would not become worn away as a result of repetitious movement of the wiper across the wiper track.
Briefly, the present invention is embodied in a potentiometer having an improved resistor card. The resistor card has generally planar conductive substrate upon which are deposited a plurality of elongate conductive lands oriented to be generally parallel to each other, each of the lands having a resistor end and a contact end. Each pair of resistor ends have a resistor between them, the resistance of which is proportional to the change of volume of fuel in the tank which corresponding to the movement of the wiper between the adjacent contact ends of the lands.
Each of the lands has a thickness with an upper surface which is raised a given elevation above the surface of the substrate. In accordance with the present invention, a substantially planar surface across which the wiper is movable. The filler consists of either an insulating material or a relatively high resistance resistive material. In the preferred embodiment, the filler has have properties which will cause it to wear at a rate which is equal to the rate at which the conductive material of the lands is worn such that the wiper track remains substantially planar during the useful life of the resistor card. The lands of a resistor card in accordance with the present invention can be made thicker than the lands of prior art resistor cards and, therefore, the resistor card of the present invention will have an extended life.